The present invention relates generally to light emitting devices, and more particularly to a display device having multiple light emitting materials deposited in a coplanar relationship to each other and method for depositing multiple light emitting materials on selected areas of the image generating surface of an electro-luminescent or electro-phosphorescent display device.
Organic Electro-luminescent and Electro-phosphorescent display devices are replacing traditional liquid crystal display devices (LCDs) in certain applications employing flat panel displays. These devices employ an organic layer of material, which emits light when current flows in the presence of a forward-bias voltage. The advantage of these devices is that unlike traditional LCDs, they are self-luminous, and thus do not require backlighting. This eliminates the need for cumbersome backlights, and thus results in a thinner, more compact display device. Electro-luminescent and Electro-phosphorescent display devices also have a wider viewing angle (up to 160 degrees), and may require less power to operate than traditional backlit LCD devices. Electro-luminescent and Electro-phosphorescent display devices are thus smaller, lighter and more efficient to use than traditional LCD devices.
The principle behind electro-luminescence is that photons are given off as electrons that are injected from a metal cathode recombine in the organic (luminescent) material with holes that are injected from an anode material. The photons are seen as light. The photons can be released from different energy states. The basic energy states being researched are the singlet, at which approximately only 25% of the maximum light is emitted, and the triplet, at which approximately 75% of the maximum light is emitted. Most of the research and development work has traditionally been done in obtaining light emission at the singlet level. Recently, however, a number of university researchers have investigated obtaining light emission at the triplet level. Ideally, obtaining light emissions from both levels would be possible, so as to achieve 100% efficiency.
There are two basic types of electro-luminescent devices. One is known as an OLED device, which stands for organic light emitting diode. OLEDs employ a small molecule system, which must be evaporated or sublimed. OLED cells comprise a stack of thin organic layers sandwiched between a transparent anode and a metallic cathode deposited on a transparent substrate. The organic layers typically comprise a hole-injection layer, a hole-transport layer, an emissive layer and an electron-transport layer. When an electric current is passed between the electrodes, the injected positive and negative charges recombine in the emissive layer to produce light (electroluminescence). The organic layers, anode and cathode are preferably selected to maximize the recombination process in the emissive layer, and thereby maximize the light output from the OLED.
The color of the light depends upon the particular type of organic material used. Different areas of the cell could have different types of organic material. The different color generating organic materials are deposited over the transparent substrate using masks. That is, separate masks are used for the deposition of red, green, and blue materials.
Polymer LEDs (PLEDs), or LEPs (light emitting polymers) as they are known, are another type of electro-luminescence device. Unlike OLEDs, which use a small molecule system, LEPs use long chain polymers. Although the properties between OLEDs and LEPs are very similar, the manner in which these systems are deposited is quite different. The organic light emitting layers of OLED devices are deposited using a vacuum deposition technique and different color OLED materials can be deposited in a pattern on the glass by using a shadow mask. The polymer light emitting layers of LEPs, however, are commonly deposited in solution, by spin coating the solution onto the surface of the glass substrate. The coating is then baked dry. A drawback of the spin coating process is that only one layer (color) can be deposited. Multiple colors cannot be photo-patterned. This is because the organic solvents used in the photo-resist and the inorganic water or solvent-based developing solutions attack the organic/polymer materials.
Accordingly, a drawback of prior art LEPs is that their deposition technique cannot generate inexpensive, multiple spatial colors, e.g., area-color or full color without the use of a secondary color filter plate. In one such prior art device, a yellow emitter, is used. While the emission spectrum of such a polymer is very broad, it has not been able to generate the entire spectrum. It is possible to obtain green, yellow, orange, and red colors from a yellow emitter by the use of a color filter, however the color blue cannot be generated. Another reason the use of color filter plates is considered undesirable is that they decrease the brightness of the secondary colors by a large amount, generally in excess of 67%. To compensate for this requires additional current and voltage to be supplied to the display, which is undesirable because additional power is consumed and the display components are placed under greater stress.
Ink-jet printing techniques have been proposed as a solution to generate high resolution, full-color displays. The drawback of this approach, however, is that the use of ink-jet printing to produce relatively large areas for each color is a relatively slow and expensive process and control of the location and amount of material has been difficult.
The present invention is directed to overcoming, or at least minimizing the drawbacks of the prior art display devices.
In one embodiment of the present invention, a method of depositing a multi-color light emitting layer over a transparent substrate used in a display device is provided. First, a first transparent conductive layer, preferably indium tin oxide, is deposited over the transparent substrate. This layer forms the anode of the emitting device. Next, a hole transport layer, preferably PEDTxe2x80x94PSS (polyethylene dioxythiophenexe2x80x94polystyrene sulphonate), is deposited over the conductive layer. This layer facilitates the communication of positive charge (electron deprived atoms) to the light emitting layer.
Next, one organic light emitting material, preferably a polymer, is deposited over one portion of the hole transport layer and another organic light emitting material, preferably another polymer, is deposited over another portion of the hole transport layer, such that the two organic light emitting materials are deposited in a coplanar relationship to each other. In other embodiments of the present invention, one or more additional organic light emitting materials are deposited over yet another portion or portions of the hole transport layer. The organic light emitting materials are preferably deposited using flexographic mats, the relief portion of which have patterns corresponding to the respective portions of the hole transport layer being covered by the organic light emitting materials being deposited. Each organic light emitting material is heated after it is deposited. The step of heating the organic light emitting materials is preferably performed in a convection oven at approximately 100 to 150 degrees Centigrade for approximately 30-90 minutes.
Next, an electron transport layer, preferably cyano PPV [poly-(cyano tere-phthalylidene)], is deposited over both light emitting materials. The electron transport layer is a layer that facilitates the communication of negative charge (electrons) to the light emitting layer. Finally, a second conductive layer, preferably a very thin layer of lithium fluoride followed by a thicker layer of aluminum, is deposited over the electron transport layer. This layer forms the cathode of the light emitting device.
All of the layers, except the multi-color light emitting layer, are preferably deposited using a flexographic printing process or a spin coating process or a vacuum deposition process or other deposition technique known in the art. Although one deposition technique is preferred to be used to deposit all of these layers, a hybrid of techniques may be used, i.e., one technique may be used to deposit one layer and another technique may be used to deposit another layer.
In another embodiment of the present invention, a display device comprising a multi-color light emitting layer deposited over a transparent substrate is provided. The multi-color light emitting layer according to the present invention includes at least two organic light emitting materials deposited in a coplanar relationship to each other. Each of at the least two light emitting materials is preferably a polymer. In other embodiments of the present invention, one or more additional organic light emitting materials are deposited in a coplanar relationship to the at least two organic light emitting materials and each other.